Write-once read-many (WORM) optical media have stamped indications of manufacturer selected laser power level for recording or writing data unto the respective media. It is a current common practice to read and use the indicated laser power level for recording on such write once media. Usually, optical disk laser controls use feedback for accurately controlling laser emission levels. As such, laser calibration upon loading a disk medium is avoided.
Because of variability between optical media, media aging and laser control circuit aging, media contamination, actual emitted laser beam power for optimum recording may be different from laser drive current settings based upon such media-indicated laser beam power level. The above-mentioned aging may vary the operation of the circuits and media response resulting in either a laser over-power level or laser under-power level situation. Such over-power laser beam levels can over ablate a track so as to obliterate adjacent recorded data, destroy a groove that interferes with tracking following and seeking, and the like. Such under-power levels may result in defective recording, such as recording over previously recorded data. Therefore, it is desired to calibrate laser power level in a write-once media recorder to avoid such under or over power laser beam levels.
In prior-art laser-beam level calibration for write-once disks used defocussed beams for avoiding ablating usable data storage space of the disk. Such out-of-focus calibration may result in a laser beam power level that is different than the calibrated laser beam level. That is, the laser beam power level for a given laser drive current changes as the beam is focused. This phenomena is caused by a shift in the differential efficiency of a semiconductor laser used in optical recording as the laser beam spot on an optical disk becomes focussed. This shift occurs because of light reflected from the disk into the laser cavity creating a cavity external to the laser. This shift in laser differential efficiency not only varies from laser to laser but also is affected by the efficiency of the optical feedback path. The path variability is caused by media variations and by contamination of the optical path (objective lens). Therefore, it is desired to calibrate and control laser beam power level in a write-once recorder using an in-focus beam and in a manner that data-storage space is not used. Further, in write-once media systems, it is not effective to use the media data-storing areas for calibrating a laser, such as is reasonable in rewriteable optical disks (usually magnetooptical). It is therefore also desired to calibrate a laser beam power level in an in-focus condition without using data fields of disk sectors that would reduce the data-storage capacity of the disk.
In many write-once optical disks, a two-byte automatic laser power correction (ALPC) field is provided in each write-once disk sector. This ALPC field enables correcting laser power for writing data at a correct or desired emitted laser power level. The ALPC field also enables the laser to be operated at write level outside of the data area. Such writing of a laser test signal in the ALPC field is monitored for ensuring that the laser is emitting a proper level laser beam to the disk. Such measurement is made with a photo detector receiving either the using so-called wasted light from a beam splitter or using light from an auxiliary port of the laser. This write testing merely turns the laser continuously on at write level for a period of time equal to scanning one or two bytes on the disk. Such an extended-time continuous write signal can have excessive energy resulting in so-called over ablation, i.e. the area ablated (physical size of the recorded laser test signal) exceeds the track width and may exceed the length of the ALPC field. Remember that such laser power level verification is measured at the output of the laser and does not measure ablation on an optical disk.
A reason for this over ablation is the duty cycle is different from a usual write pulse. That is, a usual write pulse has a duty factor of about 10% that ablates about one-half of the track width. In contrast, the 100% duty cycle used in the write qualification is extreme to often ablate radially outside of the track being written to. Such over ablation not only extends radially but also circumferentially (at the trailing edge of the DC pulse). While the duration of a recorded laser test signal need not fill the ALPC 2-byte field, many recorders do record such a laser test signal. In the latter instance, excessive laser power level results in a recorded laser test signal that crowds or extends to an ensuing write area, such as a sync area that precedes recorded user data. Recorded laser test signals having a shorter length may still radially over ablate in the ALPC area. Therefore, such extended continuous laser emissions may have undesired heating of the laser and its immediate environs. It is desired to avoid such over ablation.
The so-called correct write power level is also dependent on the duration or width of a laser write pulse. Writing in write-once media often assumes that the recorded write power level on each disk is correct. Because of circuit variations, signal delay tolerances of .+-.5%, signal propagation asymmetries in various circuits, and the like results in variations of actual recorded write pulses of a same power level that is non-linear. It is desired to avoid such unintended variations of write-once recording.